Add InlTree to the FuncInfo aux symbol in new object files.

In the linker, change InlinedCall.Func from a Symbol to a string,
as we only use its Name. (There was a use of Func.File, but that
use is not correct anyway.) So we don't need to create a Symbol
if not necessary.

Change-Id: I38ce568ae0934cd9cb6d0b30599f1c8d75444fc9
Reviewed-on: https://go-review.googlesource.com/c/go/+/200098
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>